If `vec(a),vec(b)` and `vec(c)` are non-coplanar unit vectors such that `vec(a)xx(vec(b)xxvec(c))=(vec(b)+vec(c))/(sqrt(2)),vec(b)` and `vec(c)` are non-parallel, then prove that the angle between `vec(a)` and `vec(b)` is `3pi//4` .